1. Field of the Invention
The present invention relates to the field of composite useful in the manufacture of hot melt adhesives and to the adhesive compositions themselves, and particularly to the preparation of flame retardant hot melt adhesives containing ethylene vinyl acetate.
2. Description of the Prior Art
A hot melt adhesive is generally manufactured from a mixture of three components: a thermoplastic resin capable of providing cohesion of the mixture, a petroleum resin having a tackifying effect, and a paraffin or a microcrystalline polymeric wax capable of adapting the viscosity of the mixture to the processing and use conditions of the adhesive. In choosing the thermoplastic resin part of the mixture, good properties are sought in respect to heat stability and adhesiveness. The thermoplastic resins commonly used in compositions for the manufacture of hot melt adhesives have included copolymers of ethylene and vinyl esters, particularly vinyl acetate, or copolymers of ethylene and alkyl acrylates, particularly ethyl acrylate and butyl acrylate.
A typical hot melt adhesive formulation is described in U.S. Pat. No. 4,325,853, issued to Acharya et al. on Apr. 20, 1982. This patent indicates that a typical adhesive comprises, for example, ethylene copolymer, a tackifier and a wax. The Acharya patent is directed to the particular use of alkenyl succinic anhydride in combination with ethylene copolymer and a tackifying resin. It is indicated that wax-like materials and anti-oxidants may also be used. Among the ethylene copolymers mentioned are ones with vinyl acetate or with acrylic and methacrylic acid.
A general form of hot melt adhesive is also described in U.S. Pat. No. 4,404,299, issued to Decroix on Sep. 13, 1983. The Decroix patent indicates the adhesive to conventionally include a thermoplastic resin, a tackifying resin, and a paraffin or other wax-like substance for modifying the melt viscosity. Particularly described in the Decroix patent is the use of a terpolymer of ethylene and minor parts of alkyl acrylate or methacrylate and of maleic anhydride.
While ethylene vinyl acetate (EVA) has been found to be well suited to use in hot melt adhesives, there has remained a desire to improve the flame retardancy of EVA without adversely affecting the other desirable properties of EVA. Approaches to this problem have included the modification of EVA directly or the addition of flame retardants into the overall adhesive formulation.
Hot melt adhesive formulations can be made flame retardant by the addition of halogenated liquid and/or solid additives. However, liquid additives weaken the adhesive bond strengths by dilution and plasticization of the adhesive formulation. On the other hand, solid additives act as fillers, and can result in settling and lower tensile strengths. Present adhesive dispensing systems are not generally equipped with stirrers, and solid insoluble flame retardant additives which settle out of the adhesive can clog feed lines.
U.S. Pat. No. 3,034,939, issued to Newkirk, et al. on May 15, 1962, discusses fire extinguishing properties for a hot melt adhesive composition containing chlorinated biphenyls and/or chlorinated paraffins as flame retardants. The use of the former material is now EPA regulated due to PCB contamination problems. The latter material is subject to dehydrochlorination if allowed to stand at the 270.degree.-350.degree. F. temperature required for use. Additionally, antimony trioxide and calcium carbonate (which tend to settle out) are required. There is no adhesive data in the Newkirk, et al. patent.
Other commercial flame retardants, such as decabromodiphenyl oxide (DDPO) are also used in flame retardant hot melt adhesives, as described in U.S. Pat. No. 4,727,107, issued to McConnell, et al. on Feb. 23, 1988. DDPO is an insoluble solid melting at 300.degree.-315.degree. C., which acts as a filler and can settle out of the adhesive formulation. The McConnell patent claims the use of decabromodiphenyl ether in flame retardant hot melt adhesives for polyurethane foams and fabrics. The use of antimony oxide is also claimed. These materials will settle out of the adhesive mixture. Also, formulating the adhesive is cumbersome due to the pre-melting and mixing of the flame retardant and antimony oxide with polyester to form a concentrate which is then back-added to the base polyester for application. No adhesive data is given in the McConnell patent.
The graft ethylene vinyl acetate polymers used in the present invention do not suffer the drawbacks of having insoluble flame retardants which act as fillers, increase melt viscosity, and require the use of solid antimony synergists or other flame retardant enhancing agents. Since the grafts are polymeric and compatible in the adhesive formulation, they do not perform like liquid flame retardants which can weaken and migrate from adhesive bonds.